Jahrom University of Medical Sciences Faculty of Medicine Amino Acids and Proteins

 Description – general properties Amino acids are derivatives of carboxylic acids formed by substitution of  -hydrogen for amino functional group.

majority of amino acids has amphoteric character – functional group –COOH is the reason of acidity and –NH 2 group causes basic properties. in basic environment AA dissociate proton to form carboxyl anion –COO -. Basic surround defends –NH 2 against dissociation. in acidic environment AA accept proton to form amonium cation –NH 3 +. Acidic environment defends –COOH against dissociation.

Chemical Foundations (Chapter 1) Stereochemistry ‒ Configuration & Conformation Stereochemistry: the arrangement of the molecule’s atoms in 3-D space. Configuration: Fixed structural arrangement of atoms in a molecule. Conformation: Spatial arrangements of atoms in a molecule that can come about through free rotation of the atoms about a single bond (without breaking any bonds).

R vs S D vs L cis (Z) vs trans (E) cis/trans isomers are different compounds with different properties. R/L isomers share similar chemical, but different biological properties.  Interactions between biomolecules are stereospecific. Stereoisomers: Molecules with the same chemical bonds (chemical formula), but different stereochemistry (configuration).

(2) Amino Acids: Structural Classification (Table 3-1, p. 78)

Uncommon amino acids also have important functions Residues created by modification of common residues already incorporated into a polypeptide  plant cell wall, collagen myosin prothrombim, a # of Ca + binding proteins elastin Lysine residues ~ 300 additional amino acids have been found in cells rare, introduced during protein synthesis rather than created through a postsynthetic modification

 Summary of 20 proteinaceous AA Essential AA ValinValin LeucinLeucin IsoleucineIsoleucine ThreonineThreonine LysinLysin MethioninMethionin PhenylalaninePhenylalanine TryptophanTryptophan ArginineArginine HistidineHistidine

Nonessential AA AlanineAlanine AsparagineAsparagine AspartateAspartate CysteineCysteine GlutamateGlutamate GlutamineGlutamine GlycineGlycine ProlineProline SerineSerine TyrosineTyrosine

A variety of methods have been developed Important in industry due to the commercial relevance  bodybuilding supplements (for big musles)  sources of vitamins not only for human also for animals  Synthesis of  -amino acids

 The Gabriel synthesis (from potassium phthalimide)  The Strecker synthesis  Enantioselective synthesis produces only or predominantly pure AA form. (More info Solomons & Fryhle p )

Absorption of ultraviolet light by aromatic amino acids Lambert-Beer Law log (I o /I) =  C L

 Cystine residues provide structural stability of proteins through intramolecular or intermolecular disulfide bonds. Oxidation Reduction

Non-standard (Non-canonical) amino acids: These are produced by post-translational modification, and not genetically coded, except selenocystein (Sec), which is now accepted as the 21 st amino acids.

 D-amino acids are found in a few small peptides, including some peptides of bacterial cell walls and certain antibiotics (such as penicillin). D-Glu D-Ala

Alexander Fleming discovers penicillin:

Penicillin binds at the active site of the transpeptidase enzyme that cleaves D -Ala- D -Ala and thus cross-links the peptidoglycan strands of the bacterial cell wall. Penicillin binds the enzyme by mimicking the D -Ala- D -Ala residues that would normally bind to this site. Therefore, the bacterial cell wall loses integrity and is susceptible to host defences, or simply bursts.

Dermorphin (Right) & Deltophorine (left)

(3) Amino Acid Properties: pK a, pI and Titrations In water, amino acid exists as “zwitterions” (hybrid ion).  Amino acids as acids and bases. (ampholytes)

- H hydride + 1s - Hydrogen atom Proton - Hydride, Hydrogen and Proton - Juang RH (2004) BCbasics

HighLow Proton ： abundant and small, affects the charge of a molecule H+H+ lone pair electrons H H H+H+ N H H N Amino H+H+ Ampholyte Ampholyte contains both positive and negative groups on its molecule Carboxylic C O O H C O O Proton Is Adsorbed or Desorbed pKapKa Juang RH (2004) BCbasics LowHigh pKapKa

[OH] → ★ ★ pK1pK1 pK2pK2 pH pI H-C-RH-C-R COO - NH 2 H + Isoelectric point = pK 1 + pK 2 2 Amino Acids Have Buffering Effect Juang RH (2004) BCbasics

COOH NH 2 H + COO - R - C - HR - C - H NH 2 H + R - C - HR - C - H COO - NH 2 R - C - HR - C - H Acidic environmentNeutral environmentAlkaline environment +1 0 pK 1 ~ 2 pK 2 ~ 9 Isoelectric point 5.5 Juang RH (2004) BCbasics

Environment pH vs Protein Charge + Net Charge of a Protein Buffer pH Isoelectric point, pI Juang RH (2004) BCbasics

pKa = 1.8~2.4 pKa = 3.9~4.3 pKa = 6.0 pKa = 8.3 pKa = 10 pKa = 8.8~11 pKa = 10~ H+H+ + H+H+ + H+H+ + H+H+ + H+H+ + H+H+ + H+H+ -COOH-COO - aa -COOH-COO - RR -Imidazole·H + -Imidazole His -SH-S - Cys -OH-O - Tyr -NH 3 + -NH 2 aa -NH 3 + -NH 2 RR Smaller pKa releases proton easier Residues on amino acids can release or accept protons pKa of Amino Acid Residues Only His has the residue with a neutral pKa (imidazole) pKa of  carboxylic or amino groups is lower than pKa of the R residues Juang RH (2004) BCbasics

HOOC-CH 2 -C-COOH NH3+NH3+ H HOOC-CH 2 -C-COO - NH3+NH3+ H - OOC-CH 2 -C-COO - NH3+NH3+ H NH 2 H pK 1 = 2.1 pK 2 = 3.9 pK 3 = = 3.0 first second third Isoelectric point Isoelectric point is the average of the two pKa flanking the zero net-charged form pK1pK1 pK2pK2 pK3pK3 Aspartic acid Juang RH (2004) BCbasics [OH]

Zwitterionic structure is neutral and its value of pH is called isoelectric point.

(a) Acidity and Basicity of amino acids

(b) Titration Curve of Amino Acids  The pH at which a molecule ’ s net charge is zero is called the isoelectric point or the pI  For two ionizable groups: pI = ? (such as carboxyl & amino) pK a1 + pK a2 pI = 2

A good buffer at ~ pH 6. pI =  Histidine

Serylglycyltyrosylalanylleucine. Ser-Gly-Tyr-Ala-Leu SGTAL N-terminus C-terminus

Vasopressin Oxytocin

phi (Φ) and psi (Ψ) angles

Orientation of the main chain atoms of a peptide about the axis of an α helix

Hydrogen bond formedbetween H and O atoms stabilize a polypeptide in an α-helical conformation

View down the axis of an α helix.

Spacing and bond angles of the hydrogen sheets bonds of antiparallel and parallel pleated β

A β-turn that links two segments of antiparallel β sheet.

The enzyme triose phosphate isomerase

Domain structure. Protein kinases contain two domains

Primary, secondary, and tertiary structures of collagen

Molecular features of collagen structure

Collagen

Types of collagen and their genes

Ehlers-Danlos Syndrom

Osteogenesis imperfecta

Alzheimer's disease

Beta amyloid

beta amyloid plaques in alzheimer's disease

Bovine spongiform encephalopathy

Prion